1. Field of the Invention
The present invention relates to a pneumatic booster for use in a brake system of an automobile or the like. More particularly, the present invention pertains to a pneumatic booster for use in, for example, an automotive brake system, which is so designed that it has the ability to give a driver a feeling of stable and favorable braking when the brake pedal of such a brake system is actuated.
2. Description of the Related Art
Generally speaking, in a brake system which is provided with a pneumatic booster, as the brake pedal is pressed down, the output rises in proportion to the pressing force (i.e., input force). It has been known that the driver is given a feeling of stable and favorable braking if the brake system is arranged such that, when the driver stops pressing down the brake pedal at a halfway stage (i.e., in an intermediate load state), the output is increased at a rate higher than the rate which is usually obtained in the above-described proportional relationship to the input. More specifically, when the brake is activated, a large quantity of heat is generated at the area of contact between the brake shoe and the brake drum and therefore the temperature rises to a high level, thus causing a lowering of the coefficient of friction. Accordingly, even if the brake pedal is maintained at a certain pressed position with a constant level of force, the braking efficiency deteriorates. For this reason, if the brake system is arranged such that the output is further increased even after the driver has stopped pressing down the brake pedal, it is possible to give a feeling of stable and favorable braking.
Japanese Utility Model Public Disclosure (Kokai) No. 61-64074 (1986) discloses a pneumatic booster which is designed to enable the above-described braking characteristics to be obtained.
Referring to FIGS. 12 and 13, the prior art comprises a plunger 2 which is movable in response to the movement of an input shaft 1, a first valve seat 4 formed on a valve body 3, a second valve seat 5 formed on the plunger 2, and a valve member 6 which is selectively separated from and seated on the first valve seat 4 to thereby allow a constant-pressure chamber (not shown) and a variable-pressure chamber (not shown) to be communicated with and cut off from each other, the valve member 6 also being selectively separated from and seated on the second valve seat 5 to thereby allow the variable-pressure chamber to be communicated with and cut off from the atmosphere. The valve member 6 is formed from an elastic material, for example, rubber, and orifices 6a are formed in the valve member 6 such that flow passages defined by the orifices 6a are cut off by virtue of the elastic deformation of the valve member 6 when seated on the second valve seat 5.
In operation, when the brake pedal (not shown) is pressed down, the input shaft 1 advances, and the valve member 6 is thereby seated on the first valve seat 4, thus causing the constant- and variable-pressure chambers to be cut off from each other. At this time, the second valve seat 5 separates from the valve member 6, and atmospheric air is thereby introduced into the variable-pressure chamber to effect a boosting operation.
If the driver stops pressing down the brake pedal at a halfway point (i.e., an intermediate load state), the advancement of the plunger 2 is suspended, while the valve body 3 is advanced, and the valve member 6 comes into contact with the second valve seat 5. At this time, the variablepressure chamber is communicated with the atmosphere through the orifices 6a formed in the valve member 6. As a result, the valve body 3 is gradually advanced by the action of the atmospheric air which is supplied through the orifices 6a at a relatively low flow rate, thus increasing the output. As the valve body 3 is gradually advanced, the valve member 6 is pressed against the second valve seat 5 with a force which is gradually intensified, and the valve member 6 is eventually elastically deformed to close the orifices 6a. As a result, the communication between the variable-pressure chamber and the atmosphere is cut off, and the advancement of the valve body 3 is thus suspended.
In this way, when the advancement of the input shaft 1 is suspended in an intermediate load state, the output is increased at a higher rate than the rate which is obtained when the usual proportional relationship applies, thereby giving a feeling of stable and favorable braking to the driver.
The conventional pneumatic booster suffers, however, from the following problems.
Since the orifices 6a are formed in an elastic material, for example, rubber, and they are opened and closed by making use of the elastic deformation of the elastic material, the timing at which the orifices 6a are opened or closed changes in accordance with the hardness of the elastic material. More specifically, since the hardness of the elastic material varies with any change in the air temperature and also lowers with time, it is impossible to obtain stable braking characteristics at all times.
The portion of the valve member 6 where the orifices 6a are formed is readily deteriorated because of the repetitive elastic deformation, and therefore the prior art suffers from inferior durability.